This invention relates to a multiplex transmission system for vehicles, in which multiplex transmission such as of the CSMA/CD type is applied to the transmission of signals in an automotive vehicle. More particularly, the invention relates to optimization of transmission efficiency in a network system in which communication nodes are mixed, namely high-speed communication nodes for controllers [such as an ABS (anti-lock braking system) controller or TRC (traction control) controller] that require high-speed data communication, and low-speed communication nodes for electrical accessories associated with the vehicle body or so-called "body system".
The greater use of electronics in automotive vehicles has been accompanied by a number of serious problems, among which are the increased size and complexity of the wire harnesses that interconnect the electronic components. Multiplex communication has come to the fore as a means of solving these problems, particularly in the automobile field. Multiplex communication refers to transmission, by time-division multiplexing, of plural items of data on a single wire harness, and basically employs serial transmission.
In the field of automobiles, multiplex communication networks may be considered as being divided into either of two classes, one class being complete multiplex- and partial multiplex-type networks, and the other being centralized- and decentralized-type networks.
The partial multiplex-type network mixes a non-multiplex communication portion and a multiplex communication portion. In the multiplex communication portion, switches and loads decentralized in terms of distances are connected by a multiplex transmission unit. Since separate wiring is required between the multiplex transmission unit and the switches and loads, a drawback is that the numbers of wires is greater, although the overall length of the interconnecting wiring is reduced.
The centralized-type network is one in which a plurality of slave transmission units are connected to a single master transmission unit. With a network having this configuration, a smaller harness diameter is achieved but the entire system is rendered inoperative if the master transmission unit fails. Another shortcoming is that difficulties are encountered in design modification. On the other hand, the decentralized network, though higher in cost, exhibits a number of advantages, such as a much smaller harness diameter, high reliability with regard to partial failure and greater flexibility in terms of design modification. For an example of the latter, see Japanese Patent Application Laid-Open Publication No. 62-4658.
With regard to this decentralized multiplex communication system, a CSMA/CD system is employed in accordance with an SAE (Society of Automotive Engineers) standardization proposal.
The assignee of this invention has also proposed a PALMNET (protocol for automotive local area network) system which is a further development upon this CSMA/CD system. For example, see Japanese Patent Application Publication No. 62-302421.
Further, as described in Japanese Patent Application Laid-Open (KOKAI) Publication No. 64-36541 belonging to the assignee of this invention, a master node transmits a reference pulse for time sharing the right to access a bus in order to set a channel on a transmission line.
In such a decentralized multiplex transmission system for vehicles, there are instances where communication speed is a problem. With the conventional decentralized multiplex transmission system for vehicles, no difficulties are encountered if the only data to be transmitted is that relating to simple switches and solenoids. A node for handling such low-speed accessories is referred to as a low-speed node.
However, when it is attempted to apply multiplex communication to high-speed controllers such as an engine controller, active suspension controller or traction controller, information required by these controllers is generated very frequently, and therefore communication speed becomes a problem. Nodes for handling such high-speed controllers are referred to as high-speed nodes.
In a case where a network having a low communication speed is used, there are instances where the low communication speed cannot keep up with the high speed of the controllers. In such instances, there is a high probability that the network bus will be busy, and therefore the high-speed controllers may be denied access to the bus. As a consequence, there is the danger that a delay will develop in control. When it is attempted to raise communication speed, a high-speed communication LSI is required and difficulties are encountered in terms of cost. Accordingly, when multiplex communication is applied to high-speed controllers, problems arise in respect of balancing cost and control speed.